The goals of this project are to characterize lymphocyte Fc receptors immunochemically, genetically, and functionally. Recent findings indicate that the Fc receptors of B cells and T cells but not "third population" lymphocytes nor macrophages are associated with Ia antigens. In the case of the B cell, the Fc receptors and Ia antigens appear to be distinct molecules, and there is also a single non-H-2 locus which determines antigens associated with the Fc receptor. Antigen antibody complexes can interfere with the mitogenic responses of lymphocytes. These findings indicate that Fc receptors may play a key role in Immunoregulation.